Method of stabilizing thermo-plastic containers

ABSTRACT

Thermoplastic containers, particularly polyethylene, can be stabilized with desired or required control limit on volume and shape, such that they are capable of being used at temperature ranges in excess of 110*F. without appreciable or significant loss of shape or capacity. This is achieved by placing a small amount of liquid such as water within the bottle and subjecting the bottle to a microwave radiant energy field such that the liquid (water) absorbs the radiant energy and boils off into steam. The vapor generated would appear to be converted (after all water has evaporated) into superheated vapor which scours the interior of the bottle causing it to shrink and to stabilize such that under subsequent elevated heat environments (service temperatures) it is essentially stable in volume and shape.

nited States Patent 11 1 Bazett 1 51 Feb. 13, 1973 [5 METHOD OFSTABILIZING THERMO- Primary ExaminerRobert F. White PLASTIC CONTAINERSAssistant Examiner-Richard R. Kucia [75] Inventor: Patrick SeymourBazett, London, Attorney-Philip Mnches Ontano, Canada I [57] ABSTRACT[73] Asslgnee: ggg a Llmlted' Ontario Thermoplastic containers,particularly polyethylene, can be stabilized with desired or requiredcontrol limit [22] Filed: July 15, 1970 on volume and shape, such thatthey are capable of being used at temperature ranges in excess of 110F.[21] Appl' ssool without appreciable or significant loss of shape orcapacity. This is achieved by placing a small amount CL 264/346 ofliquid such as water within the bottle and subjecting [51] Int. Cl...B29d 23/00 the bottle to a microwave radiant energy field such Fieldof Search 346 that the liquid (water) absorbs the radiant energy andboils off into steam. The vapor generated would ap- References Citedpear to be converted (after all water has evaporated) into superheatedvapor which scours the interior of UNITED STATES PATENTS the bottlecausing it to shrink and to stabilize such that 3,317,642 5/1967 Bailey..264/235 X under subsequent elevated heat environments (service2,465,130 Story temperatures) it is essentially stable in volume and3,010,157 ll/196l Cizek shape. 3,511,899 5/1970 Miller 264/26 7 Claims,9 Drawing Figures III! I 111111 11111111111111! PATENTED FEB] 3 I973SHEET 1 BF 4 FIG! IIVVfN TOR:

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PATENTEDFEBWIW 3,116,606

SHEET 2 OF 4 F|G.2A FIG.2B F|G.2C VOLUME IN GRAMS BEFORE SHRINKINGVOLUME IN GRAMS AFTER SHRINKING PATENTED FEB I 31973 SHEET 3 OF 4 mm 0EmmImSS m mwtq PATENTED FEB 1 3 I973 SHEET u 0F 4 AFTER I WASH AFTER 5WASHES AFTER IO WASHES POST OVEN CONDUCTION I l I IH I HQ 4 AGGREGATELOSS IN GRAMs OF CAPACITY AFTER Io WASHING CYCLES IHI COMPARITIVECONTROL LEVEL RANGE SHOWING Hl-LO F 5 vARIATIoNs OF CAPACITY (IN GRAMs)BETWEEN O'-IO WASHING CYCLES METHOD OF STABILIZING THERMO-PLASTICCONTAINERS This invention relates to an improved stabilizedthermoplastic container and method of making the same.

All thermoplastics have a (thermal) memory point and when they arecaused to extend beyond such (thermal) memory point they remaindimensionally stable only for an unknown and indeterminate length oftime, dependent upon the temperature to which the material issubsequently subjected. This is particularly true when servicetemperatures (temperatures to which the bottle is subjected to duringits useful life) exceed 1 F. or thereabouts.

lt is known that stress patterns which are inherently set up during theprocess of manufacture, and which create thermally unstablethermoplastic products must be substantially removed, if serviceconditions demand subjection of a moulded thermoplastic container totemperatures at or above 110F. In such processes, commonly referred toas annealing process, there is an alleviation of internal stress to alevel which creates a new point of stability above 110F., butsubstantially less than the environmental temperature to which thethermoplastic is subjected during annealing. It is for this particularreason that annealing processes are used, and why polyethylene bottlesand other thermoplastic containers are subjected to heat treatment byconduction or convection over specific time bands; namely, to attempt tostabilize the container by alleviating the internal stressescreated'during manufacture. Hot air ovens are often used in annealing aswell as water-boiling emersion and the like, controlled under acombination of temperature and related time during emersion.

It is known that thermoplastic containers or bottles, when manufacturedby the process of blow moulding, vacuum forming or other present-daytechniques, substantially reduce in capacity as the result of annealing.It is also known that stability of such annealed thermoplasticcontainers, is maintained at environmental conditions substantially lessthan the temperature employed during the annealing period, i.e. a bottleannealed by the convention method as taught by Bailey in his CanadianPat. No. 787,822 issued 18 June 1968, entitled Volume Stabilization ofMoulded Plastic Containers when subject to an annealing temperature ofup to 200F. for a time period of between 20 to 30 minutes has anempirical thermal stability level at or about 150F. When such bottle issubject after annealing to temperatures at some point above thestability level of about 150F. the bottle again is further fugitive ineither or both dimension and capacity, i.e. retained stresses arefurther relieved within the bottle, causing a fugitive reduction in itscapacity as well, in some instances, distortion of the container.

it has now been found that when thermoplastic bottles and the like aresubject to washing for the purposes of re-use or which are to be filledwith a product at about 150F., the same must be stabilized to retain thedesired capacity limits and requirements at the requisite temperaturesin excess of 150F. temperature. It has been found that thermoplasticcontainers, particularly of polyethylene, can be stabilized with thedesired or required control limits to a controlled and determinedthermal stability service level, that is to a thermal stability whichwill sustain the high environment temperature provided the servicetemperature is less than the moulding temperature of the thermoplastic.This is accomplished by subjecting a thermoplastic container, preferablya thermoplastic which is not opaque to microwave radiation selected, tothe action of microwave treatment for a substantially shorter period oftime than required by any other process taught and currently practiced.

It has been further found that an expeditious method, to achieve thepurpose of my invention, namely stabili-- ty or integrity ofthermoplastic containers, comprises the placing of a controlled amountof liquid such as water in the bottom of a thermoplastic container, suchas a polyethylene bottle (the polyethylene acting essentially as awindow" to the microwave radiation, i.e. absorbs negligible microwaveradiation) and the placing of the partially filled bottle within amicrowave radiation field, for example, that generated by microwaveovens having output in the microwave broadcast range. The contentedliquid (water) rapidly reaches its vaporization temperature, and isconverted into steam due to the absorption of radiant energy by theliquid, while the polyethylene acts substantially asa window permittingthe radiation to passthrough the container wall to the liquid. As aresult the liquid contained is elevated in temperature until it reachesbeyond its boiling point and then vaporizes off. The elevatedtemperature of the liquid, and, in its gaseous phase, the vapor phase,scavenges the inner surface of the bottle heating the same to or abovethe boiling point of the liquid, in the instance of water above 212F.when measured at sea level. The gaseous phase of the liquid also absorbsradiant energy and is further elevated in temperature (superheated) tofurther enhance the thermal scavenging of the inner surface of thebottle.

The invention therefore contemplates a method of stabilizingthermoplastic products beyond its initial or primary memory point(thermal deformation point) but below the plastic deformation pointcomprising the steps of:

a. selecting a thermoplastic product which is to be stabilized to atemperature beyond its primary memory point;

b. placing a liquid absorbant to microwave radiant energy in intimateproximity to the surface of the thermoplastic product;

c. subjecting the liquid to microwave radiant energy such that theliquid absorbs radiant energy and is elevated to a temperature in excessof the (first) primary thermal deformation point of the thermoplasticsuch that the thermo-plastic is also heated to a temperature above thatwhich created the first memory point whereby the thermoplastic isre-established into a new primary (second) memory point above the first.

The invention will now be described by way of example reference beinghad to the accompanying drawings in which:

FIG. 1 is a schematic of a production method of utilizing theembodiments of the invention.

FIG. 2 is a three-part graph illustrating the net capacity in cubiccentimeters of thermoplastic (polyethylene) containers prior toannealing and subsequent to annealing wherein the annealing processesused are two of the processes of the prior art, a 3 hour boil A, and a30 minute conduction heat shrink method C, and a third (B) the processembodying the invention.

FIG. 3A is a three part comparison graph illustrating the net capacityin grams of the containers of FIG. 2A after shrinking post boil andafter one, five and I washings for five minutes at 168F to 172F.

FIG. 3B is a three-part comparison graph illustrating the net capacityin grams of the containers of FIG. 28 after shrinking post microwave andafter one, five and washings for five minutes at l68F. to l72F.

FIG. 3C is a three-part comparison graph illustrating the net capacityin grams of the containers of FIG. 2C after shrinking post ovenconduction and after one, five and 10 washings for 5 minutes at l68F to172F.

FIG. 4 is a comparative chart showing the total capacity lost in grams(averaged) for the containers of FIGS. 2A, 2B and 2C respectively duringthe IQ washings (after the initial annealing process).

FIG. 5 is a high-low total variation chart of the capacity losses ingrams of the various containers.

It should be noted that in the above graphs, and the below describedinvention, a datum was selected as 3785 grams weight being equivalent toone United States of America gallon.

Particularly, in order to demonstrate the efficacy and the utility ofthe invention, it must be appreciated that thermoplastic bottles orcontainers which are commonly in existence in the dairy industry inNorth America are subject during their useful life span to temperaturesin excess of 150F. as a result of either dairy or consumer washingpractices. It might be mentioned in passing that the North Americandairy industry claims as a standard for dairy washing temperatures atbetween 140 to 150F. It has been found that such practices are temperedby the economies of higher wash temperature and the demands of goodsterile practices as may from time to time be prescribed by theappropriate governing bodies and agencies. As a result washingtemperatures in dairies, generally exceed the 150F. maximum, and usuallyrange in the vicinity of 165 to 170F.

As a result, if the memory point of thermoplastic diary bottles areretained in the neighborhood of I50F., as they commonly are under thepresent state of the art, and such bottles are subjected during anywashing cycle to temperatures in excess of 150F. this initiatesuncontrollable movement of the body material of the containerconstruction causing an alteration in the volumetric capacity of thesaid bottle. Such alteration of the structural and capacity integritycontinues after each washing or hot rinsing cycle, causing anunpredictable variance reflected in a new or reestablished volumetriccapacity after each wash.

Referring to FIG. 2 36 thermoplastic bottles having internal capacityslightly in excess of one United States of America gallon, were takenoff the production line immediately after manufacture thereof. Suchbottles were designed to contain an annealing allowance of approximately four ounces over the desired final capacity of one [1.8.gallon (3785 grams), all bottles were nevertheless measured for truecapacity of contents by weight. The bottles were marked, and the truecapacity of each bottle was plotted respectively on the graphs shown inFIG. 2 (solid line). The bottles were I then grouped into three equalgroups (A, B, C) and the first group, A, was subjected to a normal heattreatment in a boiling water bath, as is often practiced in the art, for3 hours at 205F. The capacity of the contents, by weight, of thesecontainers were once again taken and respectively plotted in FIG. 2A(dotted lines).

A third group of bottles was taken, group C, and the volumetriccapacity, by weight, of these bottles was measured and plotted on FIG.2C. These bottles were then subjected to heat stabilization by theconduction method as taught by Bailey in his Canadian Pat. No.

787,822 issued on 18 June, 1968, entitled Volume Stabilization ofMoulded Plastic Containers. Such bottles were subjected for a period ofthirty minutes to temperatures in the range of 198 to 205F. Afterstabilization by the Bailey method the volumetric capacity, by weight,of the respective containers was again taken and the respective valuesplotted as shown in FIG. 2C by the dotted lines.

The second group B of bottles was likewise measured, for true capacityof contents, by weight, and the same plotted on FIG. 28 (solid lines)thereafter these bottles were stabilized according to the embodiments ofthe invention now to be disclosed, in the following manner. That is tosay, 4 ounces of water were placed in each bottle and the charged bottlewas then placed in a radio frequency microwave field, microwave radiantenergy field, as generated by microwave ovens having output in themicrowave broadcast range of 890; 940; 2,400; 2,500 M. H,,. or otherapproved microwave broadcast bands. After about 30 seconds the water ineach bottle started to boil and steam could be seen coming out of thetop. After about ten more seconds steam could not be seen emanating fromthe bottle but the level of the water in the bottle could be seendiminishing in volume. After approximately one minute more or less allthe water had evaporated from within the bottle. After three minutes theradiation was turned off and the bottle removed from the oven. Aftercooling, the volume, by weight, of the bottle was again taken andrespectively plotted on the graph of FIG. 2B (dotted lines).

Now referring to FIG. 2 it can be seen that bottles subjected to thethree hour boiling, FIG. 2A, all save one, lost sufficient capacity onboiling that their net capacity was below datum (3785 grams) and in sixinstances less than l4 grams below datum (fourteen grams representingone-half fluid ounce). Referring to bottles subjected by shrinkageaccording to the embodiments of the invention it can be seen that onlythree bottles shrunk in capacity below datum and that the totaldeviation of upper limit (12 grams above datum) and lower limit (7 gramsbelow datum) was less than the deviation with respect to those bottlessubjected to the 3 hour boil, FIG. 2A. Bottles subjected to the Baileyheat conduction method of stabilization showed that four bottlesstabilized to a volumetric capacity below datum, one in excess of thefourteen grams below datum. Further, one bottle shrunk to a capacity of29 grams above datum, 28 grams representing one fluid ounce. It is to benoted at this time that in some countries, such as Canada, the governingauthorities do not permit certain containers to be on the market whichhave volumetric capacities less than one-half ounce below theirrespective datum, or in excess of 1 ounce of their respective datum.

It therefore can be seen, comparing the three techniques, that the priorart, of the 3 hour boil, FIG. 2A, and the heat conduction method ofBailey, FIG. 2C, generate bottles some of which are beyond acceptablelimits. On the other hand bottles stabilized according to the inventionall showed sufficient stabilization to be within the acceptable limitsof datum.

After stabilization all bottles were subjected to ten washing cycles induration of five minutes each at a controlled temperature of 170F.(l68l72F.). This wash did not contain any detergent or soap and might beconsidered as a hot rinse. Nevertheless, it was designed to simulateactual dairy washing cycle conditions as found in a dairy. After eachwash the capacity of each bottle was measured, and respectivelytabulated in FIG. 3 according to the method in which initialstabilization had occurred namely bottles subjected to the three hourboil method of stabilization were tabulated in FIG. 3A, bottlesstabilized according to the embodiments of the invention were tabulatedin FIG. 3B, and bottles stabilized according to the Bailey heatconduction process were tabulated in FIG. 3C. The tables only indicatethe resultant capacity after one wash, five washes and washesrespectively. In any event careful examination of the results indicatesthat the capacity of bottles, irrespective of the process ofstabilization, migrate to some extent during each wash. Comparingspecifically bottles washed according to the embodiments of theinvention, FIG. 38 with the bottles washed according to the prior art,FIGS. 3A and 3C, it can be seen that the migration of bottles washedaccording to the invention is less than migration according to the priorart. Specifically, after ten washings, bottles stabilized according tothe 3 hour boil, FIG. 3A, all, save one, reached or exceeded the lowerlimit, 14 grams below datum, and as a result were not serviceable.Moreover after the first wash all but two bottles were beyond the lowerlimit of datum.

Referring to FIG. 3C and bottles stabilized according to the Bailey heatconduction method one bottle showed migration below the lower limit ofdatum after the first wash and it remainedbelow that limit during allwashes. All other bottles seemed to be satisfactory within theacceptable limits about datum. Referring to FIG. 38 after the first washone bottle reached a capacity of the lower acceptable limit and itshowed continual reduction in capacity beyond the lower acceptable limitafter the first washing. All other bottles remained within acceptablelimits during successive washes.

Now, referring to FIGS. 4 and 5 and particularly FIG. 5 is was seen thatthe hi-low total variation which bottles migrate during the washingcycle is less with bottles stabilized according to the embodiments ofthe invention than with either the two prior art techniques. In factcareful analysis shows that bottles shrunk by the Bailey heat conductionmethod shrunk 8Q c.c. during the 30 minutes of stabilizing. After tenwashes they shrunk an additional 7 c.c. approximately. On the other handbottles shrunk according to the embodiments of the invention shrunk 106c.c. during the 3 minutes of stabilization and only approximately 3 c.c.during the 10 washings. Nevertheless it must be admitted that the totalaggregate capacity lost after 10 washings, with bottles stabilizedaccording to the embodiments of the invention is somewhat higher thanwith bottles stabilized according to Bailey, see FIG. 4, it isnevertheless less than bottles stabilized according to the 3 hour boilmethod.

From the above results it is clear that thermoplastic bottles orcontainers stabilized according to the invention are significantly morestable and have a higher integrity than those of the prior art. They arethus capable of being used in environment temperatures in the range of170F. Such containers are therefore particularly useful for food andother products which are elevated in temperature above F, in order todecrease their viscosity and to make then pourable, for

example, jellies, jams and the like, but below the thermal distortionpoint of the thermoplastic. Furthermore, dairy bottles which aresubjected to the stabilization processes embodying the invention willhave a greater inherent stability and integrity to maintain volumetriccapacity than bottles presently existing in the dairy industry.

Now referring to FIG. 1, thermoplastic bottles 10 which have just beenmoulded to an interior capacity slightly larger than the final capacitydesigned may be placed as by a hand 11, on a continuous conveyor belt12. The bottle 10, is then carried to beneath a nozzle 13, which injectsinto the bottle 10, a microwave absorbing fluid such as water 14. Thebottle 10, then progressively moves into a microwave oven 16, whichemits radiant microwave energy 17. The energy 17, passes through thebottle 10, essentially unabsorbed but is absorbed by the water 14,elevating it in temperature to its boiling point. The steam (vapor) 18,thereby generated is then superheated, scavenges the inside of thebottle 10, and shrinks or stabilizes the bottle. At the same time theexcess steam 18 (vapor) emanates from the top of the bottle pervades theinner part of the oven. As a result this steam might have somescavenging effect on the outside surface of the bottle 10, (but this isdifficult to ascertain at the present). The bottle 10, then is carriedout of the oven 16, all or some of the water having been evaporated, andthesame is then removed from the conveyor belt 12, as by hand 19 orautomatically.

It will be appreciated that though the invention has been described inone embodiment utilizing a bottle of polyethylene thermoplastic, otherthermoplastics can be used for the bottle. In such cases, thethermoplastic must have properties which permit it to act essentially asa window to the microwave radiant energy used. This allows the liquid toabsorb the radiation and thereby to be elevated in temperature.

If, on the other hand, thermoplastics which are opaque to microwaveradiation are used, i.e. thermoplastics which do not act as windows, theuse of a liquid absorbent to microwave radiation need not be ascritical.

It has been found that for the preferred results that the polyethylenebottles, above described, are satisfactorily stabilized using water asthe liquid. The amount of water to be used may vary with the size of thebottle to be stabilized; nevertheless, it would appear that for bottleshaving a volume of a pint, a minimum amount of water to be used is about1 ounce.

It might also be mentioned that a different selection of liquids withhigher vapourization temperatures will be necessary as moresophisticated thermoplastics with higher plastic deformation points areused, if it is desired to increase the range of environmental andservice temperatures for thermoplastic products.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:

l. A method of dimensionally stabilizing a thermoplastic polymericcontainer as to capacity and shape by heating the container wherein theimprovement comprises:

providing a container having an internal cavity and composed of athermoplastic polymeric material which is generally translucent to atleast one frequency of microwave radiant energy, said container having acapacity greater than the capacity to which it is to be stabilized;

introducing into said cavity a material consisting essentially of aliquid which is generally absorbent to such microwave energy to whichsaid container is translucent, said liquid having a vaporizationtemperature greater than the primary memory temperature of thethermoplastic material and less than the plastic deformation temperatureof the material,

selecting a microwave radiant energy field which has a frequency whichis generally absorbed by said liquid and to which the thermoplasticmaterial is generally translucent,

subjecting said container and said liquid to said field to elevate saidliquid to its vaporization temperature and to vaporize said liquid, soas to heat said container to a temperaturein close proximity to thevaporization temperature to establish a second memory temperature forthe container above that of the first memory temperature.

. The method of claim 1 wherein the liquid is water.

. The method of claim 1 wherein the thermoplastic is polyethylene.

4. The method of claim 3 wherein the liquid is water.

5. The method of claim 3 wherein the microwave radiant energy field isselected from the group of microwave energy frequencies consisting of890 megahertz.; 940 megahertz.; 2400 megahertz; and 2500 megahertz.

6. The method of claim 4 wherein the microwave radiant energy field isselected from the group of microwave energy frequencies consisting of890 M. H5940 M. H 2400 M. H,.; and 2500 M. H,.

7. A method of dimensionally stabilizing a polyethylene container as tocapacity and shape by heating the container wherein the improvementcomprises:

providing a container having an internal cavity and composed ofpolyethylene which is generally translucent to at least one frequency ofmicrowave radiant energy, said container having a capacity greater thanthe capacity to which it is to be stabilized;

introducing into said cavity a material consisting essentially of waterwhich is generally absorbent to such microwave energy to which saidcontainer is translucent, said water having a vaporization temperaturegreater than the primary memory temperature of the polyethylene and lessthan the plastic deformation temperature of the polyethylene;

said polyethylene having a thermal deformation temperature in excess oflF,;

selecting a microwave radiant energy field which has a frequency whichis generally absorbed by said water and to which the polyethylene isgenerally translucent;

subjecting said container and said water to said field to elevate saidwater to its vaporization temperature and to vaporize said water so asto heat said container in close proximity to the vaporizationtemperature to establish a second memory temperature of approximately F.for the container above that of the first memory temperature.

1. A method of dimensionally stabilizing a thermoplastic polymericcontainer as to capacity and shape by heating the container wherein theimprovement comprises: providing a container having an internal cavityand composed of a thermoplastic polymeric material which is generallytranslucent to at least one frequency of microwave radiant energy, saidcontainer having a capacity greater than the capacity to which it is tobe stabilized; introducing into said cavity a material consistingessentially of a liquid which is generally absorbent to such microwaveenergy to which said container is translucent, said liquid having avaporization temperature greater than the primary memory temperature ofthe thermoplastic material and less than the plastic deformationtemperature of the material, selecting a microwave radiant energy fieldwhich has a frequency which is generally absorbed by said liquid and towhich the thermoplastic material is generally translucent, subjectingsaid container and said liquid to said field to elevate said liquid toits vaporization temperature and to vaporize said liquid, so as to heatsaid container to a temperature in close proximity to the vaporizationtemperature to establish a second memory temperature for the containerabove that of the first memory temperature.
 2. The method of claim 1wherein thE liquid is water.
 3. The method of claim 1 wherein thethermoplastic is polyethylene.
 4. The method of claim 3 wherein theliquid is water.
 5. The method of claim 3 wherein the microwave radiantenergy field is selected from the group of microwave energy frequenciesconsisting of 890 megahertz.; 940 megahertz.; 2400 megahertz; and 2500megahertz.
 6. The method of claim 4 wherein the microwave radiant energyfield is selected from the group of microwave energy frequenciesconsisting of 890 M. Hz.; 940 M. Hz.;, 2400 M. Hz.; and 2500 M. Hz.